1. Field of the Invention
The present invention generally relates to a photo-setting and thermosetting resin composition, and a process for preparing a plugged-through-hole double- or single-sided printed wiring board and a plugged-through-hole multi-layer printed wiring substrate (hereinafter sometimes referred to as “a plugged-through-hole printed wiring (substrate) board”) as well as a plugged-through-hole printed wiring (substrate) board.
More particularly, the present invention relates to a photo-setting and thermosetting resin composition suitable for a plugging material for a through-hole of a double- or single-sided printed wiring board and multi-layer printed wiring substrate, a process for preparing a double- or single-sided printed wiring board and multi-layer printed wiring substrate the through-hole in which is plugged with the resin composition and a plugged-through-hole printed wiring (substrate) board prepared by the process for preparing.
2. Discussion of Background
In a conventional method of a printed wiring board, a through-hole has been temporarily plugged with a set resin, then has been subjected to an etching treatment to form a conductor circuit, thereafter a set resin has been dissolved and removed together with an etching resist by an etching resist removing liquid to expose again the through-hole.
However, if a through-hole exists in a printed wiring board, a solder flows into the through-hole and reaches an opposite surface of the printed wiring board when mounting parts such as an IC chip and so on are connected to the printed wiring board by soldering. As a result, there has been a problem that a short occurs. Further, there has been a problem that since a solder flows into the through-hole, an amount of solder required for connecting the parts lowers to cause poor connection.
Furthermore, if there exists a through-hole in a printed wiring board, when printing is made on both sides of the printed wiring board with a solder resist agent, a through-hole of the surface of the printed wiring board on which printing has been made previously is plugged in the shape of a tent with a solder resist agent; and hence the way of escape of air in the through-hole disappears when printing is made later on the opposite surface of the printed wiring board. As a result, air space sometimes remains in the through-hole. If the printed wiring board is treated with chemicals in a subsequent process under the state remained unchanged, the chemicals flow into the air space and remains therein because cleaning inside the air space can not be carried out sufficiently. As a result, there has been a problem that metal parts of the printed wiring board such as a copper-clad part, a plated part and so on corrode.
Therefore, in recent years the so-called “permanent through-hole plugging” has been carried out in which only an etching resist is removed and a set resin plugged into the through-hole is not removed. A thermosetting resin composition, which is set to form a set resin non-soluble in an etching resist removing liquid, is used as the permanent through-hole plugging material.
However, such a conventional thermosetting resin composition contains a great deal of solvent. Therefore, a great deal of solvent is evaporated when the thermosetting resin composition is thermoset and the volume of the thermosetting resin composition is shrunk sharply. As a result, the liquid level of the resin composition ((16) in FIG. 3 or (19) in FIG. 4) plugged into the through-hole of the substrate ((15) in FIG. 3 or (17) in FIG. 4) lowers; and hence a big hollow is formed or cracks are formed inside the set resin ((18) in FIG. 4).
Accordingly, when the set resin is further treated with chemicals in a subsequent process, the chemicals remain in the hollow. And sometimes the cracks inside the set resin are lengthened to the surface of the through-hole due to thermal hysteresis of the subsequent processes such as soldering and so on and the chemicals soak into inside the set resin from the cracks on the surface. These decrease the reliability or lifetime of an appliance.
Therefore, a two-stage-type thermosetting resin composition has been used as the through-hole plugging material. When the two-stage-type thermosetting resin composition is used, the primary setting is carried out at relatively low temperatures at which evaporation of solvent does not so occur to harden the shape of the resin to a certain extent and subsequently the secondary setting is carried out at high temperatures.
However, there has been a problem that while the resin composition present on the surface of the substrate among the resin compositions charged and plugged into the through-hole can be primary set at low temperatures, the resin composition present in the central region of the substrate can not be primary set sufficiently at low temperatures. In this case, if the resin composition present in the central region of the substrate is set sufficiently, the setting reaction of the resin composition present in the surface of the substrate proceeds and the hardness of the set product become hard excessively. As a result, there has been a problem that precise polishing of the surface of the substrate at a subsequent process is hard to carry out.
Therefore, in recent years a resin composition having both photo-setting and thermosetting properties (photo-primary setting+thermo-secondary setting) has been proposed instead of the above-described thermosetting resin composition which is set only by heating.
For example, JP Patent Laid-Open Publication No. 8-73565, JP Patent Laid-Open Publication No. 8-157566, JP Patent Laid-Open Publication No. 8-269172, JP Patent Laid-Open Publication No.9-107183, and JP Patent Laid-Open Publication No. 11-147285 describe a photo-setting and thermosetting resin composition.
However, each of the thermosetting resin compositions described in these publications contains epoxy rein which is solid at an ordinary temperature. When such epoxy rein is melted by heating, its viscosity generally lowers significantly. And hence, when the thermosetting resin composition plugged into the through-hole of the substrate ((20) in FIG. 5) is set by heat-setting, there is a problem that the viscosity of the thermosetting resin composition ((21) in FIG. 5) lowers and causes drips. And, when epoxy rein exists in the resin composition, there is a problem that when the resin composition is irradiated with light, light transmission is prevented due to the presence of the epoxy resin and hence effective photo-setting can not be carrier out.
JP Patent Laid-Open Publication No. 10-245431 describes a photo-setting and thermosetting sealer comprising epoxy resin, a photo-setting compound [(meta) acrylate compound and so on], a radical-generating agent and a latent curing agent.
However, in the case of the sealer, there is a problem that unset portions remain in the sealer because the setting properties of the photo-set product are not sufficient. As a result, the precise polishing of the surface of the printed wiring board in the subsequent process is hard to carry out; and hence there is a problem that a sufficient smooth surface of the substrate can not be obtained. Further, a problem occurs in chargeability and applicability of the sealer into the through-hole.
U.S. Pat. No. 2,598,346 gazette describes a photo-setting and thermosetting resin composition comprising an adduct of epoxy rein with 100% (meth) acrylic acid, a (meta) acrylate compound, a photopolymerization initiator, a thermosetting resin (bisphenol A-type resin and so on) and a curing agent (imidazole compound).
However, in the case of the resin composition, there is a problem that a photo-set product becomes hard excessively in contrast with the case of above-described sealer. Also in that case, the precise polishing of the surface of the printed wiring board in the subsequent process is hard to carry out and a sufficient smoothness cannot be obtained. As a result, there is a problem that the solder-resistance of the plugged-through-hole printed wiring board obtained is not sufficient.
U.S. Pat. No. 2,571,800 gazette describes a photo-setting and thermosetting adhesive comprising a (meth) acrylic-modified epoxy resin, an ethylene-series polymerizable compound [(meth) acrylate compound and so on], a photoradical initiator, and an epoxy curing agent (imidazole compound and so on.).
However, in the case of the adhesive, there is a case that heat-resistance is not sufficient. And hence, there is a problem that the solder-resistance of the plugged-through-hole printed wiring board prepared therefrom is not sufficient.